This invention relates to a light beam scanner in which parallelism errors of a rotary multi-surfaced mirror are corrected.
Recently, a number of devices have been developed for reading and recording information by scanning light beams, such as laser beams, and a rotary multi-surfaced mirror is typically employed as a light deflector in such devices. Parallelism errors often exist between the rotary shaft and the reflecting surfaces of such a mirror, however, even if manufactured with high precision, and such errors cause the scanning beam to be displaced perpendicular to its deflection direction. That is, the locii of the scanned light beams on the image plane, due to reflection by the various surfaces of the rotary mirror, is not a pure line but has a significant width dimension.
Some methods for forming coincident scan lines by optically correcting lateral displacement caused by parallelism errors have been proposed. For instance, in laid open Japanese Patent Application publication No. 33642/1972, correction amounts for parallelism errors are measured and stored in a memory device, and an additional light deflector is driven by signals from the memory device in synchronization with the rotation of the multi-surfaced mirror to thus eliminate lateral scan line deviations. This system is disadvantageous, however, owing to the cost and complexity of the additional light deflector, its drive circuits, and the necessary memory device.
In U.S. Pat. No. 3,750,189, two cylindrical lenses are employed. A line image is formed in parallel with a plane formed by a group of light beams deflected onto the reflecting surfaces of a rotary mirror by the first cylindrical lens, while with the second cylindrical lens the deflection point on the reflecting mirror surfaces and the scanning plane are maintained in a predetermined relationship between the object point and the image point, thereby eliminating lateral scan line deviations caused by parallelism errors. This system is disadvantageous in that it requires two cylindrical lenses, it is difficult to precisely mount them at predetermined positions, and it is also difficult to accurately manufacture such cylindrical lenses.